The present invention relates to a method and/or architecture for implementing a transmitter and/or receiver amplifier generally and, more particularly, to a method and/or architecture for implementing laser driver amplifiers that may shape or equalize the waveform of a high data rate output signal.
The need for broadband integrated circuits (ICs) that incorporate peaking control is becoming a necessity as data rates and traffic of fiber optic systems increase. Pre-emphasis or predistortion in wideband integrated circuits is useful for adjusting gain and amplitude peaking in the frequency domain. Additionally, tailoring the rising and falling edges of a signal waveform in the time domain in order to achieve lower bit error rates in fiber optic data transceiver links has also been proposed. The need for increased peaking control becomes more imperative for 10 gigabit per second fiber optic applications that require high volume high yield ICs that have a low tolerance to semiconductor process variations. Such applications include the emerging 10 gigabit Ethernet datacom system.
A broadband IC which can modulate lasers or vertical cavity surface emitting lasers (VCSELs) at data rates up to 10 Gb/s and maintain low bit error rates is coveted by engineers building high speed Ethernet systems. In practice, laser driver ICs require pre-emphasis circuits or some kind of predistortion circuit which can compensate for the distortion introduced by the nonlinear laser or VCSEL. The signal passing through a linear laser driver modulator operating at 10 Gb/s can become distorted in the process of converting from electrical to optical energy as the laser diode or VCSEL is modulated by the linear electrical driver circuit. In such a situation, the laser or VCSEL can be driven with a predistorted signal to compensate for the distortion produced by the nonlinear behavior of the VCSEL or laser.
The predistortion may be implemented by a pre-emphasis or peaking function that superimposes a weighted peaking signal on the original signal to speed up the rising and falling edges of the original data waveform. The predistortion enhances the data transition rise and fall times as well as reshapes the signal for low bit error rates (BER), inter-symbol interference (ISI), and maximum eye pattern opening.
Several conventional approaches for employing predistortion include [1] dynamic current source switching (e.g., Rainer H. Derksen, Novel Switched Current Source for Increasing Output Signal Edge Steepness of Current Switches Without Generating Large Overshoot, IEEE JSSC, vol. 30, no. 5, May 1995) and [2] pre-emphasis (digital peaking) (e.g., Ramin Farjad-Rad, et al., A 0.4-um CMOS 10-Gb/s 4-PAM Pre-Emphasis Serial Link Transmitter, IEEE JSSC, vol. 34, no. 5, May 1999). The techniques [1] and [2] are common approaches which have had practical implementations at low data rates of 2.5 Gb/s and below. However, the implementations of the techniques [1] and [2] at higher data rates is challenging. The effectiveness of the techniques [1] and [2] can be marginal at rates of 10 Gb/s and higher due to the quality of the raw data signal which provides a clock or trigger for the technique.
Additionally analog approaches do not rely on the raw data waveform as a clock or synchronizing signal for the technique to be effective. Therefore the analog approaches are more amenable to higher data rate applications. Due to the recent availability of long and short wave VCSEL technology for 10 Gb/s, an effective approach for employing pre-emphasis or peaking control for 10 Gb/s VCSEL driver applications which is amenable to high volume-high yield manufacturing is needed.
The present invention concerns an apparatus comprising a modulator circuit, a first and a second control circuit. The modulator circuit may be configured to generate a modulated differential output signal in response to a differential input signal. The first control circuit may be configured to control a first predistortion of the differential input signal in response to a first portion of the differential output signal. The second control circuit may be configured to control a second predistortion of the differential input signal in response to a second portion of the differential output signal.
The objects, features and advantages of the present invention include providing pre-emphasis and/or de-emphasis predistortion circuit that may provide (i) orthogonal pre-emphasis and de-emphasis controls for employing varying degrees of pre-emphasis and de-emphasis; (ii) duty cycle distortion correction control for recovering from undesirable duty cycle distortion produced by the pre-emphasis and/or de-emphasis control circuit; (iii) AC coupling of the peaking amplifier with a speedup capacitor and resistor which allow a degree of freedom to set up a decay time constant; and/or (iv) variability in an AC coupling tap point in order to optimize the time superposition of the pre-emphasis and/or de-emphasis signals at the output of the laser modulator driver.